The present invention relates to an electro-optical display device in which an electro-optical display cell and a dielectric sheet are layered together and in which data voltage passing through the dielectric sheet is written onto an electro-optical material layer of the electro-optical display cell.
A plasma addressed display device in which a plasma cell and the electro-optical display cell are layered together with a dielectric sheet in between, has been previously disclosed. FIG. 1 and FIG. 2 illustrate a construction of the plasma addressed display device 100.
In FIG. 1 and FIG. 2, the plasma addressed display device 100 is constructed of a flat panel in which the electro-optical display cell 1 and the plasma cell 2 are layered together with the dielectric sheet S placed between them. The dielectric sheet S is made up of laminate glass and other materials. It is necessary for such dielectric sheet 3 to be formed as thin as possible, for example into 50 .mu.m in thickness, in order to allow the display cell 1 to be driven.
The display cell 1 is comprised of an upper glass substrate (upper substrate) 4. Inside the upper substrate 4, a number of data electrodes 5 made of transparent conductive material are constructed parallel to each other at predetermined intervals in such a way as to form parallel rows. The upper substrate 4 is separated from the dielectric sheet 3 at a predetermined distance by spacers 6. The space between the upper substrate 4 and the dielectric sheet 3 is filled with electro-optical material, liquid crystal, which forms a liquid crystal layer 7. The distance between the upper substrate 4 and the dielectric sheet 3 is, For example, 4 to 10 .mu.m, and such distance is maintained equally over the display face. Material other than liquid crystal may also be used.
The plasma cell 2 is comprised of a lower glass substrate (lower substrate) 8. Inside the lower substrate 8, a number of anode electrodes 9A and cathode electrodes 9K which constitute plasma electrodes, are alternately placed so as to form columns parallel to each other and separated at a predetermined intervals. Each anode and cathode electrodes 9A and 9K has a barrier rib 10 in the middle of its upper side, which stretches along each electrodes and has a predetermined width. The tops of each of the barrier ribs 10 are in contact with the underside of the dielectric sheet 3, and this separates the lower substrate 8 from the dielectric sheet 3 by a specified distance.
A frit seal portion 11 made of low melting point glass is located on the circumference of the lower substrate 8 and combines the lower substrate 8 with the dielectric sheet 3 in an air-tight fashion. The space between the lower substrate 8 and the dielectric sheet 3 can be filled with an ionizable gas such as helium, neon, or argon, or with a mixture of such gases.
The spaces between the lower substrate 8 and the dielectric sheet 3 form discharge channels (spaces) 12 which are separated by the barrier ribs 10 and which run in rows parallel to each other. In other words, the discharge channels 12 form right angles with the data electrodes 5. Each of the data electrodes 5 acts as a column driving unit, while each of the discharge channels 12 acts as a row driving unit. Pixels 13 are provided where the discharge channels 12 intersect with the data electrodes 5 as shown in FIG. 3.
In such construction, when a predetermined voltage is applied between the anode electrode 9A and the cathode electrode 9K, which corresponds to the specified discharge channel 12, the gas in the discharge channel 12 is ionized, generating a plasma discharge, at which point the discharge channel 12 is held in anode electric potential. In this situation, when a data voltage is applied to the data electrode 5, the data voltage is written through the dielectric sheet 3 onto the liquid crystal layer 7 of each of the pixels 13, which pixels are arranged along a column corresponding to the discharge channel 12. When the plasma discharge is completed, the discharge channel 12 changes to a floating electric potential, and the liquid crystal layer 7 of each pixel 13 holds the written data voltage until the next writing period (for example, one field later or one frame later). In such process, the discharge channel 12 acts as a sampling switch, and the liquid crystal layer 7 of each of the pixels 13 acts as a sampling capacitor.
A display process is performed on a pixel basis because the liquid crystal is operated by the data voltage written onto the liquid crystal layer 7 of each of the pixels 13. Accordingly, a two-dimensional image can be displayed by scanning successively along a row the discharge channels 12 in which the plasma discharge is to be generated in order to write a data voltage onto the liquid crystal layer 7 of the pixels 13 arranged along the column.
However, when a liquid crystal display device is being driven, it is necessary to exclude a DC component in order to prevent such DC component from being applied to the liquid crystal. There is a problem in that a DC component applied to the liquid crystal causes an image sticking.
Similarly, in the plasma addressed display device described above, it is necessary to prevent a DC component from being applied to the liquid crystal 7. However, it is difficult to prevent the DC component from being applied to the liquid crystal layer 7 because of its construction. The reason is described below in reference to FIG. 4, which illustrates a circuit for one pixel equivalent to the plasma addressed display device 100.
In FIG. 4, reference character DS denotes the data voltage, reference characters RLC and CLC respectively denote a resistor and a capacitor for the liquid crystal layer 7, reference characters RG and CG respectively denote a resistor and a capacitor for the dielectric sheet 3, reference character SW1 denotes a virtual switch which is comprised of the discharge channel 12, reference character VB denotes a DC power supply, reference character R denotes a resistor for limiting a current, and reference character SW2 denotes a switch for applying a predetermined voltage between the anode electrode 9A and the cathode electrode 9K. When the switch SW2 is turned on, a predetermined voltage is applied between the anode electrode 9A and the cathode electrode 9K, the plasma discharge generated in the discharge channel 12 turns the virtual switch SW1 on, and a voltage relative to the data voltage DS is written through the dielectric sheet S onto the liquid crystal layer 7.
In this process, there are times when, due to the discharge condition, the virtual switch SW1 does not act as a simple switch, but has a direct current offset. In other words, the data voltage DS is sometimes written when the electric potential of the underside of the dielectric sheet 3 is lower than the anode electric potential, that is to say, at the middle electric potential between the anode electric potential and the cathode electric potential. This is dependent on a change in electric potential of the underside of the dielectric sheet 3 and actions caused by the discharge which reduce or weaken the charged particles (meta-stable particles).
For example, if the charged particles disappear when a electric potential difference remains as is between the anode electrode 9A and the cathode electrode 9K, the electric potential of the underside of the dielectric sheet 3 is lower than the anode electric potential, data voltage DS can be written, and the virtual switch SW1 has a direct current offset.
As described above, in the plasma addressed display device the virtual switch SW1 sometimes has a direct current offset so it is difficult to prevent the DC component from being applied to the liquid crystal, thereby a problem of image sticking arises.